Muscular weakness for voluntary movement can be extremely debilitating for hemiparetic stroke patients, yet the underlying mechanisms are largely unknown. Consequently there are few effective treatments. The primary goal of the proposed research is to characterize differences in Single Motor Unit (SMU) firing properties, specifically rate limitations and compressed recruitment range, and to correlate these changes with possible significant alterations in motor unit current-frequency relations in hemiparetic stroke subjects with muscular weakness. Fine wire intramuscular electrodes will be used to collect data from both the paretic and contralateral biceps of hemiparetic subjects. Subjects will be asked to perform constant isometric contractions at varying force levels. Force-EMG relations, MU discharge rate and recruitment profiles will be quantified and statistically compared on both paretic and contralateral sides. In order to characterize current-frequency relations, biceps tendon vibration at varying frequencies, will be applied, inducing steady state ala afferent discharge, the frequency of which is proportional to the vibration frequency. Steady state synaptic current is then broadly proportional to Ia afferent discharge frequency. Current-frequency and force-frequency relations from both paretic and contralateral sides will be quantified, analyzed and assessed for significant differences. An additional goal is to establish a relation between these MU characteristics and lesion site and lesion-volume (quantified radiologically) , addressing the possibility that pathways for specific neurotransmitters, which act as motoneuronal modulators, are involved. This would establish a foundation for potential therapeutic interventions.